Flextensional transducers

ABSTRACT

A high power, low frequency flextensional transducer (50) for underwater use comprises a number of spaced piezo-electric element stacks (53) between opposed inserts (51, 52). A Kevlar (registered trademark) compression band (54) is wound around the stacks and inserts and then partly elliptical plaster formers (56) are attached. A filament wound elliptical GRP flexural shell (57) is then wound around the assembly while controlling the tension so as to provide the required pre-stress on the piezo-electric stacks (53) when cured. After curing the plaster formers (56) are removed. End-plates (16) are attached to the elliptical shell (57) to complete the transducer; the shell (11) having a compression bonded layer (61) of neoprene applied, including a peripheral serrated lip seal (62) to seal against the end-plate (16) while permitting flexing of the shell. A device to provide wide bandwidth performance is also disclosed. To extend the range of operational depths the cavity within the transducer is filled with a gas whose vapour pressure can be temperature-controlled.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to sonar transducers and in particular toelliptical shell flextensional transducers as described in U.S. Pat. No.4,462,093 which are used to generate and radiate high power acousticenergy at low frequencies, typically in the range 200-3000 Hz.

2. Discussion of Prior Art

The construction of an elliptical shell flextensional transducercomprises fitting a piezo-electric stack (or stacks) along the majoraxis between opposing internal walls of an elliptical flexural shell incylindrical form. Each stack consists of a number of piezo-electricplates between which are sandwiched metal electrodes, these in turnbeing connected in parallel. When an alternating voltage is applied tothe electrodes a vibration is generated along the length of each stack.This vibration is transmitted to the shell and leads to an amplifiedout-of-phase vibration along the minor axis of the shell which providesthe principal motive force for this sonar transducer.

Conversely, the transducer can be used in a passive mode in whichreceived vibrations induce minor axis vibrations in the elliptical shellwhich in turn lead to electrical signals generated by the piezo-electricstacks.

The elliptical shells are commonly made of filament-wound glassreinforced plastics ("GRP"), the filament being wound around a suitablemandrel. The elliptical shell is then compressed along its minor axis bymeans of a press to permit assembly of the piezo-electric stacks alongthe major axis such that on removing the compressive force along theminor axis a residual tension remains in the shell to retain the stacksand apply a predetermined stress to the stacks. The stress applied tothe piezo-electric stack must be set to a precise value, since when thetransducer is deployed into water the increasing hydrostatic pressurewith depth reduces the stress on the stack until a limit is reachedbeyond which the elliptical shell transducer cannot be driven withoutdamage. To achieve the desired stress in the shell the piezo-electricstack output charge can be monitored at discrete conditions afterplacing shims of different discrete thickness at the end of the stack orcontinuously by using appropriately tapered wedges such as described inour copending patent application Ser. No. 8,606,745.

Flextensional transducers are normally sealed by means of end plates,however because they are capable of high power operation and thus thelarge amplitude flexing of the elliptical shell which occurs createsdifficulties in water-tight sealing between the shell and end-platessince the sealing must be effective without limiting shell movement.

In order to operate there must be a pre-stress load applied by theelliptical shell to the transducer stacks. Operation over a wide rangeof pressure-depths requires that some form of pressure-balancingarrangements is provided.

Conventional pressure compensation or balancing systems have a number ofoperational disadvantages. The most common types of pressure balancingsystems are air filled bladders and scuba type systems of which thelatter use bottled compressed air coupled to a divers pressure balancedvalve. The bladder method is severely limited as the volume of air inthe cavity of the transducer is inversely proportional to the externalhydrostatic pressure. The resulting reduction of the available sweptvolume for the active surface progressively lowers operating efficiencyas the hydrostatic pressure is increased. The scuba system is a largeand often relatively heavy appendage to a sonar transducer. In operationit can use large quantities of air if frequent changes in operatingdepth are required or if there are large unwanted depth excursions dueto the effects of ocean swell on the deployment platform.

In a conventional design of flextensional transducer the dimensions ofthe shell are calculated to utilize the first and sometimes otherflexural modes of vibration along the entire length of the ovalcylinder. The shell has therefore a single resonance frequency and afinite bandwidth associated with each flexural mode.

SUMMARY OF THE INVENTION

The principal object of the present invention is to provide anelliptical shell flextensional transducer of simpler construction thancurrently available and susceptible of easier manufacture than hithertopossible.

A further object is to provide an improved sealing between theelliptical shell and the end-plates. In addition an object of theinvention is to provide a transducer capable of operation over a widerange of pressure-depth. These, and other objects of the invention willbe apparent from the following description.

The invention provides in one form an elliptical shell flextensionalsonar transducer of the kind comprising at least one stack ofpiezo-electric elements interspersed by electrically conducting plates,the or each stack being in coplanar parallel spaced arrangement betweena pair of spaced shell inserts, the assembly of stacks and inserts beingdisposed in the plane including the major axis of a hollow flexuralshell of elliptical cross-section with the outer surfaces of the insertsin contact with the opposed inner surfaces of the shell and so shaped asto support the elliptical shape of the shell; wherein the improvementlies in:

providing a pair of resilient rectangular supports in spacedrelationship, each support being in retaining contact with the twoinserts and with one face making contact over its entire surface areawith the adjacent inner surface of the shell.

The supports are preferably so formed that when in the unstressedcondition they may be assembled with the shell inserts so as to form asupport body generally elliptical in cross-section and in conformitywith the inner cross-section of the shell.

By this arrangement the transducer may be assembled by winding aresin-coated fibre glass filament or similar composite material aroundthe support body with the piezo-electric stacks assembled between theshell inserts.

Appropriate tensioning of the filaments will lead to the desiredshell-induced stress on the piezo-electric stacks.

In one arrangement supports are sheets made of GRP and the shell insertsare provided with recesses for locating/retaining the support sheets inposition.

In an alternative arrangement the support may comprise a stifffilament-wound layer encircling the piezo-electric stacks.

In this arrangement partially elliptical formers are assembled on theouter surfaces of the layer between the spacers so as to provide theoverall elliptical former for winding on the flexural filament-woundshell.

Advantageously the stiff filament is Kevlar and the partially ellipticalformers are made of plaster so as to be removeable after forming theshell.

In an alternative form the invention provides a method of making anelliptical shell flextensional transducer comprising the successivesteps of:

a) assembling at least one piezo-electric stack between opposed shellinserts and spaced lengthwise of the inserts, and two spaced rectangularsupports between the inserts such that the inserts and the supports forma uniform cylindrical support body with an elliptical outercross-section; and

b) winding a resin-soaked filament around the assembly to form theelliptical shell.

In one aspect the support sheets and the supports and shell are madefrom GRP. Furthermore the tension in the filament wound around thesupport body assembly is preferably controlled such that the completedelliptical shell exerts a predetermined stress force along the lengthsof the piezo-electric stacks.

In another aspect the invention comprises the further step of winding alayer of a stiff filamentary material around the assembly of the or eachpiezo-electric stack and opposed shell inserts and attaching or formingpartially elliptical supports on the outside surfaces of the layerbetween the inserts, the support material being selected such that itcan be removed after winding the flexural elliptical shell. In thisaspect the layer filament is Kevlar and the supports are made ofplaster.

Advantageously there is provided a sealing member for sealing betweenthe end plates and the flexural shell, the sealing member being a lowshear modulus rubber vulcanised moulded to the outer surface of theflexural shell to form a continuous outer coating with integral lipseals on the end surfaces of the shell. Advantageously the rubber isneoprene rubber and is provided with a plurality of concentricelliptical serrations on the outer surface of the lip seal for contactwith the respective end plate. The degree of compression is ideallybetween about 10% and 30% and this determines the depth of theserrations and the dimensions of the means for holding together the endplates and shell assembly. Preferably the overall thickness of the sealis determined by the peak magnitude of the shell vibration such that thesheer stress angle is limited to 30 deg. A plurality of tie bars arefixed between the two end plates and located inside or outside the shellto determine the compression of the lip seals.

In this arrangement of the invention a method of sealing end plates to aflextensional transducer includes the steps of:

a) locating the shell on a supporting mandrel;

b) compression moulding a low shear modulus rubber coating, for exampleneoprene, over the outer surface of the shell to form a lip sealintegral therewith on each end of the shell;

c) assembling end-plates to the shell and tightening tie-bars betweenthe end plates so as to give the required compression of the end plateseals between each end plate and its respective shell end.

Advantageously the vulcanised moulding is done in a hydraulic press.During assembly of the transducer a plurality of tie-barsinterconnecting the end plates are adjusted in length to achieve thedesired compression of the lip seals.

Alternatively the serrated lip seal can be compression moulded to eachend closure plate and the complete transducer dip-coated in liquidneoprene.

For operation over a wide range of pressure-depth preferably there isprovided a pressure compensation means comprising: a cavity defined inpart by the shell of the flextensional transducer; a gas contained inthe cavity; means to vary the temperature of the gas; a depth pressuresensor; and a control circuit connected to the pressure sensor and thetemperature varying means to control the temperature of the gas suchthat the gas vapour pressure acting on the inner side of the shell issubstantially the same as the depth pressure.

In one arrangement the temperature varying means is a heating element.

The gas may fill the cavity or alternatively it may fill a bladderwithin the cavity. In a further arrangement the cavity may contain adual bladder. The gas may fill one section of the bladder and seawaterthe other section, the bladder being arranged in such a way that the gasis compressed by the external ambient hydrostatic pressure.

In the preferred arrangement the gas is dichlorodifluoromethane (freon).In addition to providing pressure compensation the gas-filled transducercan operate at a higher power duty cycle or higher ambient temperaturethan hitherto possible. Waste heat generated in the activepiezo-electric elements of the transducer is transferred away moreefficiently by the dichlorodifluoromethane and other similar suitablegases than by the conventionally used air or nitrogen. Suitable gasesare those which have a convenient vapour pressure temperaturecharacteristic. Thus these transducers can operate at greater depth thansimilar current transducers before thermal runaway.

In order to provide broad-band operation the two inserts located one ateach end of the major axis between the shell wall and the correspondingend of the transducer stack and generally "D" shaped in cross section tomaintain the elliptical shape of the shell may be formed such that thearcuate length of each insert surface in contact with the shell wallchanges along the length of the shell cylinder.

In one form there may be one or more discrete length changes of thearcuate surface of each insert. By this means there are produced two ormore regions along the length of the shell having differing free lengthsof vibrating shell. Advantageously the shell is segmented along itslength with weakened regions corresponding to the positions of changingcross section of the inserts. By this means a number of discretefundamental flexural mode resonances can be excited by driving thepiezo-electric stack assembly at these frequencies with the weakenedportions assisting towards decoupling the different length portions ofthe shell.

In another form wherein the shell is uniform along its length thearcuate profile of each insert cross section is progressively changedalong the length or part of the length of the shell.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only withreference to the accompanying Drawings of which:

FIGS. 1 and 2 show side elevation and plan cross-sections through aconventional elliptical shell flextensional transducer;

FIG. 3 shows a perspective view of the support body for an ellipticalshell flextensional transducer;

FIG. 4 shows the cross-section through the flextensional transduceraccording to the invention;

FIG. 5 shows a perspective view of an alternative arrangement offlextensional transducer.

FIG. 6 is a cut-away view of a shell/end plate sealing arrangement;

FIG. 7 is a modification of the FIG. 2 arrangement to provide depthcompensation;

FIG. 8 shows the vapour pressure vs temperature characteristic ofdichlorodifluoromethane;

FIG. 9 shows an alternative vapour control mechanism for extending thedepth capability of the transducer;

FIG. 10 is a perspective view of a further form of flextensionaltransducer; and

FIG. 11 is a perspective view of an alternative arrangement to FIG. 10.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

The flextensional transducer shown in FIGS. 1 and 2 comprises afilament-wound GRP flexural shell 11 of an elliptical cylindrical forminto which a piezo-electric stack 12 (or stacks) is fitted along themajor axis of the ellipse. The stack 12 consists of a number ofpiezo-electric plates 13 between which are sandwiched metal electrodes14 connected in parallel. "D" section end members 15 are provided tolocate the ends of the stack 12. Steel end plates 16 are provided toclose the ends of the elliptical shell 11 thereby defining a transducercavity 17. The cavity 17 may be filled with dichlorodifluoromethane aswill be described later. The elliptical shell flextensional transduceris used to generate and radiate high power acoustic energy at lowfrequencies, typically from 200 to 3000 Hz. The transducer is operatedby applying an alternating voltage to the electrodes which causesvibrations to be generated in the directions 18 along the piezo-electricstack 12. These vibrations are transmitted to the elliptical shell 11and lead to increased amplitude vibrations in the directions 19 on theminor axis of the shell. Conversely the transducer can be operated in apassive mode when pressure fluctuations in the surrounding medium 20lead to vibrations in the directions 18 along the stack 12 which in turnlead to an alternating output signal from the transducer electrodes 14.

One arrangement according to the present invention shown in FIGS. 3 and4 shows two shell inserts 31 and 32 provided along their upper and loweredges 33 and 34 with recesses extending along their lengths so as toprovide seats for two curved rectangular resilient GRP support sheets 35and 36. When assembled as shown in FIG. 3 the outer surface conformssubstantially to the inner surface of the elliptical shell 37 of theflextensional transducer to be made.

Each transducer stack 38 is assembled in spaced relationship one toanother between end plates 39 and 40, and the end plates are in contactwith the inserts 31 and 32. The surfaces of the end plates 39,40 and thecontacting surfaces of the inserts 31,32 are radiused to improve thealignment of the transducer stack assembly. Support plates 35 and 36 areplaced in position within the recesses 33,34 in the shell inserts 31,32as shown in FIG. 3. The GRP shell 37 is then filament-wound around theinserts and the support sheets. Means common in the art is provided toadjust the tension of the filament such that the overall tension exertedon the piezo-electric stacks 38 by by the completed elliptical shell 37reaches the design value. This may be monitored by taking repeatedreadings of the outputs of the piezo-electric stacks 38.

FIG. 5 shows an alternative arrangement of the flextensional transducer50. Shell inserts 51 and 52 are assembled with a number of spacedtransducer stacks 53 of which one is shown. A pre-stress compressionband 54 of filament-wound Kevlar (Registered Trade Mark) is appliedaround the stack assembly. Partially elliptical plaster supports 55, 56are then attached to the outer surfaces of the Kevlar band such that thecomplete assembly provides an elliptical former around which the GRPelliptical shell 57 can be wound. The Kevlar compression band 54 andassociated plaster supports 55, 56 provide sufficient support to ensurethat the elliptical shell 57 is wound with the required tension forcorrect operation of the transducer. After curing of the shell 57 theplaster supports 55, 56 are removed. The glass-resin system used to makethe shell 57 is selected to retain a high residual stress during curing.

Once the stack-shell assembly is completed end plates are attachedhaving a bonded serrated neoprene seal. The complete transducer is thendip coated in liquid neoprene.

The invention described greatly facilitates the manufacture of thetransducer compared with the conventional techniques since there is norequirement for the use of a high pressure press to extend the majoraxis of a pre-manufactured shell in order to insert the piezo-electricstack assemblies. In addition there is no need for carefully machinedwedges to adjust the tension force in the shell acting along the lengthsof the piezo-electric stacks.

FIG. 6 shows the sealing arrangement between the elliptical GRP shell 11and one of the steel end plates 16. The shell 11 has a bonded neoprenecoating 61 on its outer surface and integrally formed therewith is anend seal 62 bonded to the end face 63 of the shell 11. The end seal 62is formed on its outer surface, adjacent to the steel end plate 16, withconcentric serrations 64 running around the elliptical seal. A pluralityof tie rods 65 are connected between the end faces and, on assembly ofthe transducer, the lengths of the tie rods are adjusted to determinethe required compression of the end seal between the end plates and theshell. The degree of compression is determined by the depth of theserrations in the seal. Compressing the rubber reduces its shear modulusthereby enhancing acoustic decoupling. The overall thickness of the sealis determined by the peak magnitude of the shell vibration and therequirement to limit the sheer stress angle to 30 deg.

The neoprene coating 61 and lip seals 62 are compression bonded to theGRP shell 11 in the following way. After being treated with appropriatebonding preparations, the shell is placed on a support mandrel, enclosedin a steel mould, and the neoprene compression moulded and bonded to theshell in a heated platen hydraulic press. An opening 66 is provided forentry of an electrical cable to the transducer stacks.

The water integrity of the seal has been tested to a hydrostaticpressure of 2 MPa and dynamically tested at full power for 350 hours. Inaddition access to the inside of the transducer, for example, forreplacing piezo-electric stack elements.

In an alternative arrangement the serrated lip seal may be compressionbonded to each end plate 16 and the complete assembly then dip coatedwith a sealing agent, advantageously liquid neoprene.

In the arrangement shown in FIG. 7 attached to one end plate 16 withinthe cavity 17 is a thermostatically controlled heater 71 controlled by aunit 72 outside the cavity. The unit 72 includes a pressure transducerfor measuring the pressure of the ambient medium 70 and a controlcircuit to provide suitable temperature control signals to thethermostatic heater 71. Details of the unit 72 are not shown since theywill be readily apparent to those experienced in this field.

FIG. 8 shows the variation with temperature of the vapour pressure ofdichlorodifluoromethane measured in feet of water. The control circuitregulating the setting of the thermostatic heater 71 acting on thedichlorodifluoromethane is arranged to match the pressure within thecavity 17 to the hydrostatic pressure of the surrounding medium 70. Bythis means the tension in the flexural shell 11 is maintainedsubstantially constant and the piezo-electric elements act under thesame operating conditions throughout a wide range of pressure depths.Dichlorodifluoromethane has a relatively low vapour pressure at ambienttemperatures and a vapour pressure of 250 PSIA at 65° C.

In addition to providing a relatively simple pressure compensatingmechanism, the use of gases similar to dichlorodifluoromethane in placeof the conventionally used air or nitrogen helps to control thedissipation of waste heat. Heat generated by the active elements of thetransducer during high power operation can lead to thermal runaway undersome operating conditions with air or nitrogen filled cavities. Althoughthe thermal conductivity of dichlorodifluoromethane is less than air ornitrogen it has a higher heat capacity and lower gaseous viscosityleading to a higher heat transfer capability and improved heatdissipation capability when used in sonar transducers. This enables thetransducer to operate at a higher power duty cycle or higher ambienttemperature and hence greater operating depth without thermal runaway.

A further advantage results from the increased insulating effect withincreased depth of the dichlorodifluoromethane and similar gases. Inmany conventional high power transducers the factor limiting the rangeof use is the breakdown voltage of the cavity medium at the appliedelectric field. Transducers filled with these gases generatingrelatively high internal depth compensation pressures could therefore besubjected to a greater electric field and hence generate more power.

As an alternative to filling the cavity 17 directly with gas a bladderfilled with the gas may be provided inside the cavity 17. Thermostaticcontrolled heating of the gas would then be carried out inside thebladder. Alternatively the gas may be used to fill one section of a dualbladder inside the cavity of the transducer 17. The other section of thebladder would then be filled with seawater by providing a conduitconnected to external seawater at ambient hydrostatic pressure.

In an alternative arrangement closed or open cycle refrigeration systemsmay be coupled to the flextensional transducer to control the pressureof a refrigerant gas inside the transducer. A simplified system isillustrated in FIG. 9 wherein the interior of the flextensionaltransducer shell 90 is included in a refrigeration loop including acompressor 91 and a condenser 92. A control system (not shown) isrequired to start the compressor 91 when the pressure difference betweenthe seawater and the refrigerant was lower than required, and to actuatethe throttle valve 93 allowing vapour to enter the shell 90 from thecondenser 92 in the converse situation. The condenser 92 thus acts as arefrigerant reservoir. A stop valve 94 is included in the line betweenthe condenser 92 and the transducer 90. In order to operate with arefrigeration system the initial bias stress of the elliptical shellmust be arranged such that the vapour pressure variation achieved by therefrigeration equipment maintains the bias stress on the piezo-electricstacks within design limits.

FIG. 10 shows a flextensional transducer modified for broadbandoperation. The elliptical shell 101 is GRP as before but its outersurface is formed with two grooves 102 transverse to the shell length onthe lower surface as well as the upper surface as shown. The outerportions 103 and 104 of the insert 105 have their edges 106,107 cut awaywith the edges of the cut-away portions corresponding approximately tothe positions of the shell grooves 102. The grooves 102 extendsubstantially as far as each fulcrum 108, 109 and may be formed bysawing substantially through the shell. As shown the cut-away edges 106,107 result in the fulcra 108, 109 of the end portions 103, 104 of theshell being displaced from the fulcrum 1010 of the centre portion 1011of the insert. The effective beam length of the centre portion of theshell 1011 is thus less than the effective beam length for the outerportions of the shell. By segmenting the shell in providing theweakening grooves 102 each segment is partly decoupled from the adjacentsegments and thus the beam can be made to vibrate at more than onefundamental flexural mode resonance on excitation by driving thepiezo-electric stack 1012 at these frequencies.

The number of segments can be larger than three and each segment couldhave a different effective beam length by appropriate forming of theinserts 105. Typical frequency variations of ±30% from a mean value offlexural resonance have been achieved with the present invention. Theradiated power in each component can be predetermined. It has been foundthat this is related to the dimensions of the radiating surface and tothe flexural resonant frequency. Thus the disposition of the segmentscan be arranged to enable the shape of the acoustic power frequencyresponse to match a required characteristic. For example the segmentscan be arranged to reduce the peak power and widen the effectiveband-width.

FIG. 11 shows an alternative embodiment of the invention. In this formthe elliptical shell 111 is uniform along its length withoutsegmentation. In place of the step-wise change of profile of the insertas in FIG. 10 there is a gradual change along the length of the insertsuch that the effective beam length is a maximum at each end of theshell and a minimum at the centre. This is done by a gradual cut-away atthe top and bottom edges of the insert 112 from zero at the center 113to a maximum at the ends 114. With sufficient lateral decoupling in theGRP shell 111 there will be a consequential gradual change in flexuralresonance along the length of the shell. Although the FIG. 11arrangement is shown such that there is symmetry about the centre of theshell, other gradual changes of the effective beam length may be used asfor example by gradually increasing the effective beam length throughoutthe length of the shell.

Modifications of the invention will be apparent to those skilled in theart all falling within the scope of the invention defined herein.

We claim:
 1. An elliptical shell flextensional sonar transducer of thekind comprising:a hollow flexural shell of elliptical cross-section; apair of spaced shell inserts; at least one stack (12) of piezo-electricelements (13) interspaced by electrically conducting plates (14), saidat least one stack being in coplanar parallel spaced arrangement betweensaid pair of spaced shell inserts (15), said stacks and inserts beingdisposed in the plane including a major axis of said hollow flexuralshell (11) with outer surfaces of the inserts in contact with theopposed inner surfaces of the shell and so shaped as to support theelliptical shape of the shell; and a pair of resilient rectangularsupports (35, 36, 55, 56) in spaced relationship, each support being inretaining contact with the two inserts and with one face making contactover its entire surface area with the adjacent inner surface of theshell.
 2. A flextensional transducer as claimed in claim 1 wherein thesupports (35,36,55,56) are so formed that when in the unstressedcondition they may be assembled with the shell inserts (31,32,51,52) soas to form a support body generally elliptical in cross-section and inconformity with the inner cross-section of the shell.
 3. A flextensionaltransducer as claimed in claim 2 wherein the rectangular supports aresheets (35,36) made of glass reinforced plastic and the shell inserts(31,32) are provided with recesses for locating/retaining the supportsheets in position.
 4. A flextensional transducer as claimed in claim 2wherein the rectangular supports comprise a stiff filament-wound layer(54) encircling the piezo-electric stacks together withpartially-elliptical formers (55,56).
 5. A flextensional transducer asclaimed as claimed in claim 4 wherein the stiff filament is Kevlar andthe partially elliptical formers are made of plaster so as to beremoveable after forming the shell.
 6. A flextensional transducer asclaimed in claim 1 wherein there is provided a sealing member (62) forsealing between the end plates and the flexural shell, the sealingmember (62) being a low shear modulus rubber vulcanised moulded to theouter surface of the flexural shell to form a continuous outer coatingwith integral lip seals (64) on the end surfaces of the shell.
 7. Aflextensional transducer as claimed in claim 6 wherein the rubber isneoprene rubber and is provided with a plurality of concentricelliptical serrations (64) on the outer surface of the lip seal forcontact with the respective end plate.
 8. A flextensional transducer asclaimed in claim 7 wherein the degree of compression of the lip sealbetween the shell and the lip seal is between 10% and 30%.
 9. Aflextensional transducer as claimed in claim 7 wherein the thickness ofthe seal is such that the sheer stress angle is limited to 30 deg.
 10. Aflextensional transducer as claimed in claim 6 wherein a plurality oftie bars is fixed between the two end plates (16) and located inside oroutside the shell to determine the compression of the lip seals.
 11. Aflextensional transducer as claimed in claim 1 wherein there is provideda pressure compensation means comprising:a cavity defined in part by theshell of the flextensional transducer; a gas contained in the cavity;means (71,92) to vary the temperature of the gas; a depth pressuresensor (72); and a control circuit; the control circuit being connectedto the pressure sensor and the temperature varying means to control thetemperature of the gas such that the gas vapour pressure acting on theinner side of the shell is substantially the same as the depth pressure.12. A flextensional transducer as claimed in claim 11 wherein thetemperature varying means is a heating element.
 13. A flextensionaltransducer as claimed in claim 11 wherein the gas fills the cavity. 14.A flextensional transducer as claimed in claim 11 wherein the gas fillsa bladder within the cavity.
 15. A flextensional transducer as claimedin claim 11 wherein the cavity contains a dual bladder, the gas fillingone section of the bladder and seawater the other section; the bladderbeing arranged in such a way that the gas is compressed by the externalambient hydrostatic pressure.
 16. A flextensional transducer as claimedin claim 11 wherein the gas is dichlorodifluoromethane.
 17. Aflextensional transducer as claimed in claim 1 wherein the two insertslocated one at each end of the major axis between the shell wall and thecorresponding end of the transducer stack and generally "D" shaped incross section to maintain the elliptical shape of the shell are formedsuch that the arcuate length of each insert surface in contact with theshell wall (103,104,105) changes along the length of the shell cylinder.18. A flextensional transducer as claimed in claim 17 wherein there areone or more discrete length changes of the arcuate surface (103,104,105)of each insert.
 19. A flextensional transducer as claimed in claim 18wherein the shell is segmented along its length with weakened regions(102) corresponding to the positions of changing cross section of theinserts.
 20. A flextensional transducer as claimed in claim 17 whereinthe shell is uniform along its length the arcuate profile of each insertcross section is progressively changed along the length or part of thelength of the shell (113,114).
 21. A method of making an ellipticalshell flextensional transducer comprising the successive steps of:a)assembling at least one piezo-electric stack (38,53) between opposedshell inserts (31,32,51,52) and spaced lengthwise of the inserts, andtwo spaced rectangular supports (35,36,55,56) between the inserts suchthat the inserts and the supports form a uniform cylindrical supportbody with an elliptical outer cross-section; and b) winding aresin-soaked filament around the assembly to form the elliptical shell(37,57).
 22. A method as claimed in claim 21 wherein the supports aresheets (35,36) and the supports and shell (37) are made from glassreinforced plastic.
 23. A method as claimed in claim 21 wherein thetension in the filament wound around the support body assembly iscontrolled such that the completed elliptical shell exerts apredetermined stress force along the lengths of the piezo-electricstacks.
 24. A method as claimed in claim 21 wherein there is includedthe further step of winding a layer of a stiff filamentary material (54)around the assembly of the or each piezo-electric stack (53) and opposedshell inserts (51,52) and attaching or forming partially ellipticalsupports (55,56) on the outside surfaces of the layer between theinserts, the support material being selected such that it can be removedafter winding the flexural elliptical shell.
 25. A method as claimed inclaim 24 wherein the layer filament is Kevlar and the supports are madeof plaster.
 26. A method of making a flextensional transducer as claimedin claim 6 including the further step of sealing end plates to theflextensional transducer comprising:a) locating the shell (11) on asupporting mandrel; b) compression moulding a low shear modulus rubbercoating (61), for example neoprene, over the outer surface of the shellto form a lip seal (62) integral therewith on each end of the shell; c)assembling end-plates (16) to the shell and tightening tie-bars betweenthe end plates so as to give the required compression of the end plateseals between each end plate and its respective shell end.
 27. A methodas claimed in claim 26 wherein the compression moulding is done in ahydraulic press.
 28. A method as claimed in claim 26 wherein duringassembly of the transducer a plurality of tie-bars interconnecting theend plates are adjusted in length to achieve the desired compression ofthe lip seals.
 29. A method as claimed in claim 26 wherein as a finalstep the complete transducer is dip-coated in liquid neoprene.